limit test of libxml-specific DocumentFragment#dup behavior

... to only running when testing the libxml2 impl.

Related to #1846 and #1063.

test/xml/test_document_fragment.rb

@@ -263,11 +263,13 @@ def test_issue_1077_parsing_of_frozen_strings
         Nokogiri::XML::DocumentFragment.parse(input) # assert_nothing_raised
       end
 
-      def test_dup_should_exist_in_a_new_document
-        # https://github.com/sparklemotion/nokogiri/issues/1063
-        original = Nokogiri::XML::DocumentFragment.parse("<div><p>hello</p></div>")
-        duplicate = original.dup
-        assert_not_equal original.document, duplicate.document
+      if Nokogiri.uses_libxml?
+        def test_dup_should_exist_in_a_new_document
+          # https://github.com/sparklemotion/nokogiri/issues/1063
+          original = Nokogiri::XML::DocumentFragment.parse("<div><p>hello</p></div>")
+          duplicate = original.dup
+          assert_not_equal original.document, duplicate.document
+        end
       end
 
       def test_dup_should_create_an_xml_document_fragment